In painting, the colour purple was traditionally achieved by artists mixing red and blue pigments together. Different mixtures resulted in purples of varying intensities and shades.
But how can the separate ingredients of a purple be identified?
At the National Gallery Scientific Department a tiny sample of paint, usually taken from the edge or from underneath the frame of a picture, is immersed in resin, dried, and then polished to produce a cross-section.
This cross-section can be examined at high magnification under a microscope.
This sample clearly shows separate blue and red pigment particles. Other methods of examination tell us even more.
This scanning electron microscope, coupled with an energy dispersive x-ray analyser, can determine the composition of the pigments. The blue pigment in this purple paint contains copper, identifying it as the mineral azurite. The red pigment contains aluminium, which is characteristic of what are known as ‘red lake’ pigments.
The colour in red lake comes from the dyestuff in insects such as kermes or cochineal, or plants such as madder root. Analysing a red lake using high performance liquid chromatography can identify which of these materials were used. To understand the ways in which a colour could have been obtained, members of the Department use historical recipes to prepare pigments.
This is a 19th century method for obtaining red lake from madder root.
Red lake pigments are particularly prone to fading on exposure to light.
In this portrait of a young princess by Jan Gossaert the blue pattern on the sleeves was originally purple. The red lake pigment Gossaert mixed with blue azurite has almost entirely faded and the original colour is now only visible at the edge, where the paint has been protected from light.
The Scientific Department’s contribution to understanding the materials, behaviour and stability of colour helps conservators and curators not only understand how the paintings in the collection have changed over time,
but also how best to preserve them for the future.